Various expansion processes are known for natural gas liquids (NGL) recovery, and especially for the recovery of propane from high pressure feed gas. Most conventional high propane recovery processes are complex in design, typically requiring propane refrigeration and turbo expanders for feed gas chilling, column reflux, and at least two fractionation columns (e.g., absorber, demethanizer, and/or deethanizer). While such known processes can achieve over 95% propane recovery, cost and energy consumption are generally very high. Additionally, pipeline operators may desire to reserve some propane in the residue gas to improve the heating value of the pipeline gas, and therefore not always opt for high propane recovery. In such cases, medium propane recovery processes (e.g., propane recoveries of 50% to 80%) are more economical.
To reduce at least some of the capital and/or operating expenses, propane refrigeration requirements can be reduced by cooling the feed gas in a demethanizer overhead exchanger and/or one or more side reboilers to partially liquefy the feed gas. The so formed liquid phase of the feed gas is then separated from the vapor phase, which is typically split in two streams. One stream is further chilled and fed to the upper section of the demethanizer as reflux while the other stream is letdown in pressure in a turbo-expander and fed to the mid section of the demethanizer. For propane recovery, a second column (e.g., deethanizer) is then used that receives and separates the demethanizer bottoms into an ethane overhead and the desirable propane product. Such configurations typically require costly processing equipment and considerable horsepower to compress the residue gas from the demethanizer to pipeline pressure, thereby rendering such plants often uneconomical.
Alternatively, high propane recovery can be achieved by recovering propane content in the residue gas from the demethanizer column by operating the demethanizer at a relatively low temperature, or by adding an additional rectification stage. Lower temperatures can be achieved by further lowering the demethanizer pressure at the expense of even higher residue gas compression horsepower. On the other hand, where a relatively high feed gas pressure is present, the demethanizer column pressure could theoretically be increased to thereby reduce residue gas compression horsepower and thus lower the overall energy consumption. However, the increase in demethanizer pressure is typically limited to between 450 psig to 550 psig as higher column pressure will decrease the relative volatilities between the methane and ethane components, making fractionation difficult, if not even impossible.
Exemplary NGL recovery plants with a turbo-expander, feed gas chiller, separators, and a refluxed demethanizer are described, in U.S. Pat. No. 4,854,955 to Campbell et al. Here, a configuration is employed for NGL recovery with turbo-expansion, in which the demethanizer column overhead vapor is cooled and condensed by an overhead exchanger using refrigeration generated from feed gas chilling. Such additional cooling step condenses most of the propane and heavier components from the demethanizer overhead, which is later recovered in a separator and returned to the column as reflux. The demethanizer bottoms is fractionated in a deethanizer, which is refluxed with propane refrigeration. Unfortunately, while such processing steps significantly improve the propane recovery to over 95%, the energy consumption is relatively high. Similar configurations are shown in WO 99/30093, WO97/16505, WO 2005/045338 A1, WO 02/14763 A1, and WO 03/100334 A1 with similar difficulties.
Thus, while numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering propane and heavier natural gas liquids from natural gas and other sources, all or almost all of them suffer from one or more disadvantages. Most significantly, heretofore known configurations and methods are costly (operating and/or capital cost) and often complex and energy intensive. In addition, conventional methods of demethanization typically fail to exploit the economic benefit of high feed gas pressure. Therefore, there is still a need to provide improved methods and configurations for natural gas liquids recovery, especially where the feed gas pressure is relatively high.